Taxol is a microtubule agent isolated from the stem bark of Taxus brevifolia, the western (Pacific) yew tree. Taxol acts by promoting the formation of unusually stable microtubules, inhibiting the normal dynamic reorganization of the microtubule network required for mitosis and cell proliferation (Schiff, P. B., et al. (1979) Nature 277, 665; Schiff, P. B., et al. (1981) Biochemistry 20, 3247). In the presence of Taxol, the concentration of tubulin required for polymerization is significantly lowered; microtubule assembly occurs without GTP and at low temperatures, and the microtubules formed are more stable to depolymerization by dilution, calcium, cold, and inhibitory drugs. Taxol will reversibly bind to polymerized tubulin, and other tubulin-binding drugs will still bind to tubulin even in the presence of Taxol.
Taxol interacts with the microtubule system of many types of organisms. For example, in mammalian cells a 50 Nm Taxol concentration usually causes a significant increase in microtubule number, with changes in cell shape and mitotic arrest in actively dividing cells. (Parness, J., et al. (1982) Biochem. Biophys. Res. Commun. 105, 1082). These perturbations of microtubule function caused by Taxol have a critical impact on the cell because of the role played by microtubules in cell motility, secretion, and cell division.
Taxol has been studied for its effect in combating tumor growth in several clinical trials using a variety of administration schedules. Severe allergic reactions have been observed following administration of Taxol. However, it has been demonstrated that the incidence and severity of allergic reactions is affected by the rate of Taxol infusion (Weiss, R. B., et al. (1990) J. Clin. Oncol. 8, 1263).
Cardiac arrhythmias are associated with Taxol administration, and like allergic reactions, their incidence is affected by the rate of Taxol administration. Sinus bradycardia and Mobitz II arrhythmia will develop in approximately 40% and 5% of patients, respectively, beginning 4-6 hours after the start of a Taxol infusion, and continuing for 4-8 hours after its completion. In most patients, the abnormal rhythm is hemodynamically stable and does not require cardiac medications. Additionally, it has been observed that the incidence of severe cardiac events is low in patients receiving Taxol alone. Thus, infusion times up to 24 hours have been used in treatment with Taxol to decrease the incidence of toxicity and allergic reaction to the drug. Data from these studies indicates that reversible myelosuppression is the dose limiting toxicity, with significant peripheral neuropathy observed at doses of 275 mg/M.sup.2 and greater. Other toxicities include myalgia, mucositis, and alopecia.
Clinical studies of Taxol have been performed in a number of tumors including acute leukemias, breast cancer, ovarian cancer and melanoma. In one study of 34 patients with ovarian cancer treated with 250 mg/M.sup.2 Taxol as a 24 hour continuous infusion, there was a 21% objective response rate (Enzig, A. I. (1990) Proc. AACR 31, 187). The major toxicities were neutropenia and peripheral neuropathy. Another study of 30 patients with melanoma treated with the same dose and schedule of Taxol exhibited an objective response rate of 13% (Enzig, A. I. (1988) Proc. ASCO 7, 249).
Rowinsky, E. K., et al. ((1989) Cancer Res. 49, 4640) describes a phase I study of Taxol in 17 patients with refractory acute leukemia. Taxol was administered as a 24 hour continuous infusion and escalated from 200 to 390 mg/M.sup.2. Severe mucositis limited further dose escalation, and other nonhematological effects included peripheral neuropathy, alopecia, myalgias, arthralgias, nausea, vomiting, and diarrhea. Based on this study, the maximum tolerated dose and recommended phase II doses for Taxol were 390 and 315 mg/M.sup.2, respectively. Nine patients had transient reductions in peripheral blood and bone marrow blasts, and three patients had complete clearance of leukemia for less than one month.
Two mechanisms of Taxol resistance have been identified in vitro. In one type of cell, resistance is on the basis of drug efflux, and like other multidrug-resistant (mdr) cell lines, it has increased levels of membrane P-glycoproteins(s), and shows increased drug efflux (Gupta, R. S. (1985) Cancer Treat. Rep. 69, 515). These cells are also resistant to the vinca alkaloids, doxorubicin, and other natural products, and resistance is reversible with calcium channel blockers such as verapamil (Racker, E., et al. (1986) Cancer Treat. Rep. 70, 275). Another mechanism of resistance found in other taxol-resistant cells involves mutations in the alpha- or beta-tubulin subunits, with some of these cell lines actually requiring taxol for growth and mitotic spindle formation (Schibler, M. J., et al. (1986) J. Cell Biol. 102, 1522).
A potentially important factor in the area of Taxol research involves the multidrug resistant gene (mdr). The product of this gene, called the p170 glycoprotein, has been demonstrated in vitro to play a role in resistance of tumor cells to Taxol. It has been demonstrated that p170 functions as a membrane pump which actively transports intracellular drug out of the cell. Using in vitro cell models, several classes of drugs have been identified which block the action of p170, including cyclosporin A, calcium channel blockers, phenothiazines, and anti-arrhythmic drugs such as quinidine and amiodarone. These drugs have been shown to reverse mdr-induced drug resistance by increasing intracellular drug concentrations.
Investigators have found increased expression of mdr in a variety of tumor types, including lymphoma and breast cancer. For example, Goldstein, L. J., et al. ((1989) J. Nat'l Com. Inst. 81, 116) reports moderately increased mdr expression in 22% (4/18) of untreated lymphomas and in 60% (3/5) of treated lymphomas. Although it is unknown if increased mdr expression is a clinically relevant mechanism of drug resistance, there is in vitro evidence that mdr plays a role in drug resistance to Taxol.
Although it is clinically unknown whether Taxol, an antimicrotubule agent effective in the treatment of cancer, is less effective in cells expressing the mdr gene, in vitro data shows that this is a concern. Furthermore, no one has been able to demonstrate an effective regimen for treatment of lymphomas and breast cancer which overcomes the problem of mdr drug resistance. The present invention discloses a method of treating breast cancer with a low-dose long term exposure to Taxol. There is clinical and laboratory evidence that long infusion times, on the order of 72-96 hours, may enhance the activity of drugs, such as Taxol, which are transported by P-glycoprotein. (Lai, G. M., et al. (1991) Int. J. Cancer, 49, 696). Thus the method of the present invention is to administer taxol as a 96 hour infusion in patients with breast cancer, to effectively treat the disease and potentially reduce the chances of developing mdr Taxol resistance.
Accordingly, an object of this invention is to provide a method of Taxol treatment effective against breast cancer.
An additional object of this invention is to provide a method of Taxol treatment which reduces or eliminates the development of mdr Taxol resistance.